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| Diatoms |
Diatoms are unicellular microorganisms of the phylum Bacillariophyta that are abundant in aquatic, semi aquatic, and moist habitats throughout the world, growing as solitary cells, chains of cells, or members of colonies.
Diatoms, algal organisms of the phylum Bacillariophyta, have more than 250 genera and about 100,000 species. A distinctive siliceous cell wall called a frustule surrounds each vegetative cell. Diatoms have an extensive fossil record, going back some 100 million years to the Cretaceous period.
Deposits of fossil diatoms, known as diatomite or diatomaceous earth, are mined commercially for use as abrasives and filtering aids. One subterranean marine deposit in Santa Maria, California, is about 900 meters in thickness. More than 270,000 metric tons of diatomaceous earth are quarried annually from a deposit in Lompoc, California.
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Analysis of fossil diatom assemblages provides important information on past environmental conditions. Although the ancestry of diatoms is obscure, they share similar pigments, food reserves, and plastid structure with the chrysophytes and complex, multicellular brown algae.
Classes
Diatoms are frequently separated into two classes based on differences in the symmetry of the cell wall. Centric diatoms (class Centrobacillariophyceae) may be circular, triangular, or rectangular but typically have surface markings that radiate from a central area, an arrangement called radial symmetry.
Pennate diatoms (class Pennatibacillariophyceae) are elongated with surface markings at right angles to the long axis, an arrangement called bilateral symmetry.
Cell Wall Components
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| Cell Wall Components |
An organic layer composed of carbohydrates and amino acids covers the siliceous frustule. The frustule is composed of two halves that fit together like the plates of a petri dish. The larger, overlapping half, the epitheca, sits atop the smaller hypotheca.
The epitheca is composed of a relatively flat upper part (valve face) with downturned edges (valve mantle), called the epivalve, and one or more hooplike girdle bands called the epicingulum. Similarly, the hypotheca is composed of a hypovalve and a hypocingulum. The epicingulum and hypocingulum are collectively known as the girdle.
When viewed with the microscope, frustules may be seen in two very different perspectives, depending on the position in which they are lying. If the valve face and outline of the valve are visible, the frustule is said to be in valve view. If the valve mantle and girdle are visible, the frustule is said to be in girdle view.
The varied shapes and beautiful ornamentation of these walls give the diatom cell
its intrinsic beauty and have long been used to classify diatoms. Many pennate diatoms possess a raphe, a slit along one or both valves, divided by a thickened bridge of silica (a central nodule) and terminated by polar nodules.
Raphid pennate diatoms are capable of gliding movement that is caused by the secretion of mucopolysaccharides, derived from vesicles or crystalline bodies, through the raphe. Araphid pennate diatoms lack a raphe and posses a central, unornamented area known as the sternum or pseudoraphe. No centric diatom has a raphe.
Simple pores (puncta), more complex areolae, or chambers (loculi) are frequently arranged in regularly spaced lines (striae) which, in turn, may be strengthened by siliceous ribs (costae). Loculi and areolae open externally by a delicate pore plate and internally by a large, round hole.
Because the protoplast is completely enclosed by the frustule, the flux of materials across the cell wall primarily occurs through these pores and slits. Additional processes and appendages may extend from the valves of different species.
Electron microscopical studies have revealed two additional openings in the valve: the fultoportula (strutted process) and the rimoportula (labiate process). The fultoportula consists of a tube that penetrates the wall and is supported internally by two or more buttresses. Fultoportulae are confined to a single order of centric diatoms.
The rimoportula, found in centric and pennate diatoms, consists of a tube that opens to the outside by a simple aperture and internally by a longitudinal, liplike slit. Some researchers suggest that the raphe system may have evolved from one or more rimoportulae.
Reproductive Strategies
The most common mode of reproduction in diatoms is asexual by cell division of a diploid vegetative cell to produce two daughter cells. Following mitosis the protoplast expands, pushing apart the valves, and divides by furrowing.
Each daughter cell receives the epitheca of the parent and forms a new hypotheca within a silica deposition vesicle. The daughter cell that receives the original epitheca remains the same size as the parent.
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| Reproductive Strategies |
However, the daughter cell that receives the original hypotheca forms a new hypotheca and is usually smaller than the parent cell. Thus, the average cell size of a population of diatoms may become progressively smaller during the growing season. The maximum size of the population is restored during sexual reproduction.
During sexual reproduction, diploid vegetative cells divide by meiosis to form haploid gametes (eggs and spermin centric diatoms or amoeboid gametes in pennate diatoms). Fusion of the gametes results in a diploid zygote that enlarges to several times its original size by the uptake of water and forms valves.
This enlarged zygote (auxospore) has a different valve morphology from that of the valves of vegetative cells. Auxospores, which do not serve as resting spores, divide by mitosis to form vegetative cells with the maximum size for the species.
Some centric diatoms form resting spores in response to the availability of various nutrients (especially nitrogen), temperature, light intensity, and pH. Resting spores are short cells with thick walls that differ fromthewalls of vegetative cells.
Resting spores are usually formed within the frustule of a vegetative cell. A vegetative cell may give rise to one, two, or four resting spores. Resting spores germinate in light when environmental conditions improve.
Flagella
Flagellated cells are found only in the male gametes (spermatazoids) of some centric diatoms. Each spermatozoid bears a single flagellum covered with stiff tubular hairs (mastigonemes). Based on ultra structural studies of two species, it appears that diatoms lack the normal 9 + 2 arrangement of microtubules in the shaft of the flagellum (axoneme).
The two central microtubules are missing, leaving nine peripheral doublets (9 + 0). Furthermore, the basal body of these species consists of nine doublets of microtubules, instead of the normal nine triplets found in other eukaryotes.
Food Reserve
The most important carbohydrate food reserve is the chrysolaminarin, a beta-1,3-linked glucan, which is stored in special vacuoles. Chrysolaminarin (a water-soluble polysaccharide) is also the most important food reserve in the brown algae and chrysophytes.
Diatoms may also accumulate various lipids. The fatty acid composition of these lipids differs somewhat from that found in the green algae and higher plants, notably in the absence of linolenic acid from most species. Lipids may be stored within or outside vacuoles.
Photosynthetic Pigments
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| Photosynthetic Pigments |
The photosynthetic pigments are stored within membrane-bound organelles called plastids. Many pennate diatoms have two large plastids, while centric diatoms generally have a large number of small discoid plastids. Four membranes surround the plastids of diatoms: a double-membrane envelope and a layer of endoplasmic reticulum that is continuous with the nuclear envelope.
Each plastid containsmore or less parallel lamellae composed of three stacked, flattened sacs (thylakoids) and at least one pyrenoid, which does not appear to be directly associated with any food reserve product.
Habitat
Diatoms are usually a major component of benthic and planktonic communities in all but the hottest and saltiest waters.While most diatoms live in water, a few species grow in damp soil and can tolerate extreme drought and heat for some time.
Pennate diatoms are common members of the benthos and plankton in marine and freshwater habitats,while centric diatoms are more commonly found in the marine plankton. Some benthic diatoms grow attached to rocks, sand grains, other algae, aquatic plants, and even animals.
Other benthic diatoms (usually raphid pennates) live freely on the surface of, or in, the substrate. Planktonic diatoms often produce spring and fall blooms in temperate lakes and oceans and summer blooms at higher altitudes.
In coastal waters and lakes, they may produce resting spores to survive between growing seasons. One estimate suggests that 20-25 percent of the total primary production on earth is contributed by marine planktonic diatoms.
